With the rapid progress of the communication technology, in the future 3G network architecture, a mobile network is no longer limited to a circuit switching mode, but has gradually evolved to an internet protocol (IP) network. As we step into the R5 stage of the 3rd Generation Partnership Project (3GPP), a core network of the universal mobile telecommunications system (UMTS) is divided into three subsystems, namely, a circuit switched (CS) domain, a packet switched (PS) domain, and an IP multimedia subsystem (IMS).
The IMS is a subsystem superimposed on the existing packet domain in the wideband code division multiple access (WCDMA) network which is newly added in 3GPP R5 stage. The main functional entities in the IMS include: a call session control function (CSCF) entity, adapted to provide a user registration control function and a session control function; an application server (AS), adapted to provide a logic function of controlling various service; a home subscriber server (HSS), adapted to provide a centralized management of subscribers' subscription data; and a media gateway control function (MGCF)/IMS Media Gateway (IM-MGW) entity, adapted to realize an intercommunication with the circuit switching network. Furthermore, the CSCF may be divided into a proxy CSCF (P-CSCF), a serving CSCF (S-CSCF), and an inquiring CSCF (I-CSCF). A terminal equipment (TE) accesses the IMS through the P-CSCF of the local place where the TE is located currently. Session and service trigger control and interaction with the AS service control are implemented by the home domain S-CSCF of the place where the TE is registered, and the I-CSCF is adapted to realize (for example, inquiry) the route from the P-CSCF to the S-CSCF.
The IMS is at the core control layer, and an access network for accessing the IMS network may be any IP access network, including a WCDMA network, code division multiple access 2000 (CDMA2000) network, wireless local access network (WLAN), high rate packet data (HRPD) network. Alternatively, an asymmetric digital subscriber line (ADSL) network and a cable network can also access the IMS. The core control layer provided by the IMS has nothing to do with the accessing motion.
With the development of the communication network, the IMS will coexist with the existing 2G network represented by a global system for mobile communication (GSM) and CDMA for a long time. During this time period, the IMS mainly covers the regions with a high demand for data services in large and medium-sized cities, and the IP access networks exist as hot islands or isolated islands in the sea of 2G or 3G networks. After a TE enters a certain IP access network and gains an IP connection, it registers for a contact address (an IP address assigned by the IP access network) with the IMS network. After the network and the TE have authenticated each other successfully, the current IP address binds with one or more common subscriber identities (IDs) of a subscriber of the TE. After the IMS registration, the TE can initiate a call through the common subscriber ID, or receive a call from other subscriber terminals. When an IMS registrar, i.e., the S-CSCF, receives a session invitation from other TE to the current TE, that is, a target of the received invitation is the common subscriber ID of the current TE, the IMS registrar extracts an IP address corresponding to the common subscriber ID according to the previous binding, replaces the target in the session invitation with the IP address, and sends the session invitation to the current TE through the IP access network. When the TE leaves the IP access network, the TE initiates deregistration to realize unbinding of the current IP address of the TE and the common subscriber ID. After the TE has performed the IMS un-registration (deregistration), the IMS registrar no longer sends a session invitation to the TE through the IP access network.
The process of the TE for initiating a session initiation protocol (SIP) registration with the IMS through a UMTS network is as shown in FIG. 1. The TE initiates the registration through a PS accessing part of the UMTS network (including two parts, namely, CS and PS). The TE first acquires an IP address in the PS access network, and sends a register message REGISTER (sip: xxxxx.com SIP/2.0, To:<sip:Walter®xxxxx.com>, Contact:<SIP:[5555::1:2:3:4]>; expires=7200) according to the acquired IP address. In the register message, the sip:xxxxx.com indicates that the final destination of the message is a registrar. The header To indicates a common subscriber ID to be registered (for identifying a subscriber, and it is an account in a communication network). The header Contact indicates the IP address of the TE in the UMTS access network. The parameter “expires” indicates the time duration that the TE wants to register (7200 s, i.e., two hours), and if the parameter “expires” is set to 0, it indicates that the TE wants to perform an IP address deregistration. The above SIP registration aims at informing the registrar: the common subscriber ID can be visited via a contact address (the IP address) indicated in the header Contact. The register message is sent to the S-CSCF through the P-CSCF and the I-CSCF, and an authentication for an initial registration of the TE is performed by the S-CSCF, thereby completing the registration.
After the registration is completed, if the TE or the P-CSCF wants to acquire more messages of the subscriber registration, it sends a SUBSCRIBE message to the S-CSCF to subscribe the subscribers' registration state. After performing the subscription, whenever the registration state of the TE is changed, the S-CSCF sends the registration state of the TE to another TE or the P-CSCF through a NOTIFY message, as shown in a broken-line frame in FIG. 1.
After the TE has successfully registered for the contact address with the IMS network, the TE can initiate a re-registration at any time by sending a new REGISTER request to the IMS network. For example, since the timeout occurs to the registration time, the registration needs to be refreshed, and in this case, it needs a re-registration. The processing of the re-registration is completely the same as the initial SIP registration, but the bi-directional authentication between the TE and the network is not necessary.
The deregister process of a registered contact address may be initiated actively by the TE when leaving the IP access network, and a network side S-CSCF may also un-register (i.e., deregister) the registration of a subscriber whenever needed, for example, when a pre-paid TE uses up the balance when making a conversation, the IMS network deregisters the registration of the subscriber. When the IMS network needs to perform the subscriber deregistration, the S-CSCF sends a deregister notification to the TE and the P-CSCF through a NOTIFY message, and the NOTIFY message indicates which common subscriber ID is to be deregistered, as shown in FIG. 2.
The processes of registration, re-registration, and deregistration for a common subscriber ID performed by a TE have been briefly illustrated above, and a voice call continuity (VCC) is introduced below. The VCC is at first proposed by the 3GPP and aims at solving the continuity problem between a circuit voice call of a circuit switching (CS) network and a voice over IP (VOIP) call based on the IP access network, which enables a TE to switch conveniently between the CS network (the CS part of the GSM or UMTS) and a WLAN access network.
The VCC technology is based on the IMS. In order to support the VCC, after a TE has completed the registration, the S-CSCF needs to perform an initial filter criteria (iFC) check on a network element VCC AS (introduced by the 3GPP2, and currently there are four independent functional blocks in the 3GPP) based on a subscriber allocation downloaded from the HSS. After the VCC AS passes the check, the S-CSCF performs a third-party registration with the VCC AS to notify the VCC AS that the TE has performed the IMS registration, and subsequently, the call of the TE can be received and delivered through the IP access network, as shown in FIG. 1. Likewise, after the TE is deregistered, the S-CSCF needs to perform the iFC check on the VCC AS based on the subscriber allocation downloaded from the HSS. After the VCC AS passes the check, the S-CSCF performs a third-party registration with the VCC AS to notify the VCC AS that “the TE has cancelled the IMS registration”, and subsequently, the call of the TE cannot be received and delivered through the IP access network any more, as shown in FIG. 2.
When another TE initiates a call to the TE through the CS network, the signaling needs to enter the IMS network, the VCC AS is responsible for selecting a delivering domain for the call, i.e., selecting to deliver the call through the IP access network where the TE registers or through the CS network, and the network side delivers the call to a target TE through the selected IP access network or the CS network.
However, the inventors found that, when the TE leaves the IP access network abnormally, for example, the TE loses signals of the currently connected IP access network, but does not have enough time to deregister the contact address to the IMS network, when a call corresponding to the common subscriber ID is received. Subsequently, it is possible to still deliver the call through the IP access network, thereby causing a call delivery failure or a prolonged call establishing time.